# B Why does light move slower in a more dense medium?

1. Feb 4, 2017

### instantresults

As I understand it light is refracted when entering a different medium (like from air to glass to air) because it moves slower through the glass, so it will go through the glass in as least time possible , so it bends toward the normal so it can traverse the glass quickly (if thats right word that is).
But why does it actually go slower through the glass? Is it like the photons are bumping into more obstacles in a denser medium so it takes longer time to pass through? thanks for any help

2. Feb 4, 2017

### Svein

1. Check out https://www.physicsforums.com/threa...e-that-the-speed-of-light-is-constant.533832/
2. From https://en.wikipedia.org/wiki/Speed_of_light: The speed at which light propagates through transparent materials, such as glass or air, is less than c; similarly, the speed of radio waves in wire cables is slower than c. The ratio between c and the speed v at which light travels in a material is called the refractive index n of the material (n = c / v). For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at c / 1.5 ≈ 200,000 kilometres (120,000 mi) /s; the refractive index of air for visible light is about 1.0003, so the speed of light in air is about 299,700 kilometres (186,200 mi) /s (about 90 kilometres (56 mi) /s slower than c).

3. Feb 4, 2017

### TeethWhitener

I think instantresults was asking specifically why the refractive index of materials differs from 1. In the wiki page that Svein linked to, the section called "Microscopic explanation" gives a pretty good layperson's description of the process.

4. Feb 4, 2017

Staff Emeritus
Before determining why something is true one should determine if it is true. It is true that light travels more slowly in a medium than in vacuum. But does the speed of light (or equivalently, the index of refraction) depend on density? Let's look at the data.

5. Feb 4, 2017

### instantresults

The linked wiki page states that
"..However, this represents absorption and re-radiation delay between atoms, as do all slower-than-c speeds in material substances. ..."these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrarily later time, as stimulated by a second laser pulse. During the time it had "stopped," it had ceased to be light. This type of behaviour is generally microscopically true of all transparent media which "slow" the speed of light"

is this (basically) referring to how photons excite the electrons in atoms into higher energy states, and then they fall back to the lower energy states? If that is true then wouldnt that mean that the more electrons there are for a single photon to interact with the longer it would take for that energy to be re-emitted? thanks again for the help

6. Feb 4, 2017

### sophiecentaur

This is far too simple a model. If it were just a matter of each photon being absorbed and then being re-radiated by a single atom the result would be radiation in all directions because there is a random delay in that process. The way to look at it is that the incident WAVE interacts with the material as a WHOLE. It is a coherent process. Steer clear of Photons except when they are relevant.

7. Feb 4, 2017

### ZapperZ

Staff Emeritus
This explanation isn't correct. There are two examples to counter this:

1. Consider diamond and graphite. Both consists of predominantly carbon. Yet, their optical properties are VERY different. For the same type of atoms that form these two material, the index of refraction (signifying the speed of light in the material) of graphite is different than diamond. So if all this is is simply the absorption of the atoms that make up the material, why are these two carbon material have such different optical properties. This shows how the carbon atoms are arranged to make up each of these material can play a significant role in the optical property, and why solid state physics is not the same as atomic physics.

2. Consider the existence of birefringent material. These are materials that have different optical properties along different crystallographic directions. The speed of light in these material is different in one direction than in another direction. Again, if it is all simply the excitation of atoms that make up the material, there shouldn't be any influence in the direction of light propagation.

Zz.

8. Feb 4, 2017

### Cutter Ketch

To be clear I am sure vanadium is disputing only the OPs questioning supposition that perhaps the index of refraction is related directly to density. Asking why light moves slower in a medium is a perfectly reasonable question.

9. Feb 4, 2017

### sophiecentaur

The circumstance where it is correct is where you have a low density gas, in which the atoms are so spaced out that they interact with one photon per atom (or molecule) Under those circumstances, you can get re-emission in all directions.

Last edited: Feb 6, 2017
10. Feb 4, 2017

### phinds

Just so you're clear, that makes no sense at all. It is trivially easy to construct an irregular volume of glass for which the light bending, as it will, in accordance with the refractive index, will cause the beam to take much longer to make it through the volume. The light beam cannot possibly "know" in advance what the shortest path would be so that cannot be any part of the reason why it bends as it does.

11. Feb 4, 2017

Staff Emeritus
Which is why I wrote

12. Feb 4, 2017

### Cutter Ketch

There are a couple of different ways to think about this question. You can think of it as the electrons and nucleii responding to the applied oscillating field. In an applied electric field the material will polarize, electrons being pushed one way and the nucleii being pushed the other. In an oscillating field the polarization field will try to oscillate along with the applied field. Because the charge is bound and also because the charge carriers have mass the polarization of the material cannot keep up with the applied field. The polarization is retarded in phase relative to the applied field. The propagating field is the sum of the applied field and the polarization field so as the light propagates through the medium the speed of the net wave is slowed. How much the light is slowed is determined by the magnitude of the polarization and how much the phase of the polarization field lags behind the the applied field. Those things are determined by how the charge is bound in the material and how it is free to respond to the field. That does depend on the density of charge available to polarize, but it depends much more critically on the states available for the electrons. If the field were in a direction and at a frequency that the polarization of the charge put the electrons in an allowed energy state, then the light would be absorbed. The incident field would be resonant with an allowed transition. Well, just like driving a tuned oscillator away from its resonance, the closer you are to resonance the higher the amplitude and the larger the phase lag. That is why the index of refraction gets larger as the energy of the photon gets larger and therefore closer to an allowed absorption (typically the band gap of the optical material, but it works the same way near discrete absorption frequencies)

Another way to think of it which will bug some people is in terms of virtual absorptions and the Heisenberg uncertainty principle. The electrons in the material will have allowed absorptions. A photon with an energy not equal to the allowed transition can still interact with the electron. The electron can be promoted to an energy state which is not allowed, but due to the Heisenberg uncertainty principle, the uncertainty in the energy of this virtual state includes the allowed transition for a period of time. When enough time passes that the uncertainty in energy becomes smaller than the difference between the virtual state energy and the allowed state energy the photon must be reemitted. The closer the energy of the virtual state is to the allowed state the longer the virtual state lives and the larger the phase delay and the index of refraction. Some have suggested that such a description can't work because the electron would emit in all directions. This is incorrect. The virtual state is created with the phase of the incoming photon. It takes time for the electronic state to lose this phase relation through random processes. The allowed lifetime for the virtual state is shorter than the dephasing time and the reemited photons have a calculable phase delay which in the same way that the phase relations had all the original photons traveling in the same direction requires that all the emitted photons continue in the same direction. This quantum mechanical description can be made to reproduce all of the observed behavior and is mathematically equivalent to the more classical wave description, so your choice of description is a matter of philosophy.

13. Feb 4, 2017

### Cutter Ketch

Actually, come to think of it the difference in index of refraction between specifically air and glass IS to some large extent due to the density. Air has a LOT less charge to polarize per unit volume than glass. You know for example that the index of refraction of a gasses below high pressures does in fact vary linearly with density. Also you know in general gasses all have very low indices of refraction compared to almost all solids and liquids. Yes the bonds have different transition dipole moments, and, yes without a band structure there is effectively a lot less freedom for the charge to interact, and yes the refraction will depend highly on the proximity to absorptions, etc. But you can't discount density particularly when comparing gasses to solids. Solids tend to have similar charge density, so the density is less critical than the electronic structure. But if you REALLY change the density, then it has to count in some significant way.

14. Feb 6, 2017

### Khashishi

I was going to say something about polarization, but it looks like Cutter Ketch already explained it quite well. So, instead, I'm going to say something about the particle aspect. When light is traveling through vacuum, it is just an electromagnetic wave. And photons are the quanta of the quantized electromagnetic wave. But the electromagnetic field is coupled to the polarization and magnetization of the matter. So, some fraction of the energy and momentum of the light wave is in the electromagnetic field, and some fraction is in motion of the atoms. So, the propagating wave is not purely an electromagnetic wave. The quanta of this hybrid wave perhaps could be thought of as a quasiparticle, related to the photon, but with a different mass. Since it has mass, it travels slower than c. Experts: is that correct? Is the quanta related to a polariton?

15. Feb 6, 2017

### phinds

Uh ... what atoms would that be ?

16. Feb 7, 2017

### Khashishi

When I say "But the electromagnetic field is coupled to the polarization and magnetization of the matter." I'm talking about light traveling through a medium.

17. Feb 7, 2017